The present invention relates to the field of pollution control, and specifically to electrostatic divices for removal of particles, such as dust or aerosols, from the air.
Devices for electrostatic dust suppression have been known for some time. U.S. Pat. No. 4,335,419 gives an example of such a dust control apparatus. The disclosure of the latter patent is incorporated by reference herein. In devices of the prior art, as exemplified by the disclosure in the cited patent, a stream of liquid is atomized in a nozzle, producing a large number of tiny droplets. The droplets pass through a charging ring, disposed around the nozzle. The ring is connected to a power supply, which electrically charges the ring. The ring induces a charge, of the opposite polarity, on the droplets leaving the nozzle.
The mechanism by which the above-described devices operate is not fully understood. It has been suggested, in the cited patent, that the charged droplets attract oppositely charged particles of dust or other fine pollutants, causing the pollutants to precipitate out of the air. The latter explanation is not wholly satisfactory, because it fails to explain why such devices tend to remove pollutants regardless of the polarity of their charges. A more probable mechanism is the creation of a "space charge" by the charged mist. By some means, possibly through free ions in the air, a charge of the same polarity as the space charge is transferred to the pollutant particles. The pollutant particles are therefore repelled from the space charge, in all directions. Any initial charges on the particles are overwhelmed by the greater space charge, so that their initial polarity becomes insignificant.
The first electrostatic dust suppression devices available on the market used bare metal electrodes as charging elements. But a bare metal electrode is a safety hazard. If handled while the unit is turned on, the electrode can cause an electric shock. Although the currents used in such devices are generally not sufficient to produce a lethal shock, such shocks can cause severe injuries indirectly. For example, the shock victim who is standing atop a tall ladder is likely to sustain severe injuries in a subsequent fall.
A bare metal electrode is likely to cause frequent electric discharges, similar to those created by a Van de Graff generator. These discharges are dangerous in an explosive environment. They have also been known to cause the devices to catch fire.
One possible alternative to a bare metal electrode is to use a less conductive material as the charging element. Thus, some of the electrostatic dust suppression devices of the prior art have employed a plastic material, such as phenolic, as the charging element. Phenolic is normally an insulator at commonly-used voltages, of the order of 120 volts. But at voltages substantially in excess of 1000 volts, phenolic will transmit a significant amount of leakage current. At these higher voltages, phenolic can function effectively as an inductive charging element.
But the use of phenolic as the material for the charging element creates new problems. When the phenolic has absorbed substantial amounts of moisture, its resistivity decreases further. The high votage, which is used to charge the liquid droplets, can "leak" along unexpected and undesired paths. Absorption of moisture thus can short-circuit part or all of the charging voltage to ground, impairing or destroying the effectiveness of the unit. The short-circuit due to moisture creates an additional shock hazard, And, as stated above, the device, in operation, constantly handles a stream of water, so the problem of moisture adsorption is always present, regardless of the external atmospheric conditions.
One can remove moisture from the phenolic electrode by baking it at high temperature, and then coating it with several layers of a material such as epoxy. This procedure eliminates the moisture, but does not always prevent its later reabsorption. The procedure also adds to the cost of making the device.
The present invention provides a device for suppressing dust and other pollutants, which does not require a bare metal electrode, and which also works reliably and safely at any level of humidity. With the device of the invention, it is not neccessary to bake, or otherwise specially treat, the charging head to drive off moisture. The invention employs a charging head made of at least two distinct materials, the materials having different resistivities, to provide adequate charging capability and also to maintain safety of operation.